MappReduce实现大小排序

Mapreduce中的默认排序规则是按照key值进行的,如果key为封装Int的IntWritable类型，那么MapReduce按照数字大小进行排序，如果key是封装String的Text类型，那么map reduce按照字典顺序进行排序。Reduce自动排序的数据仅仅是发送到自己所在节点的数据，使用默认排序的时候并不能保证全局的顺序，这是因为在排序之前还有一个partition的过程，默认无法保证分割后各个Reduce上的数据整体是有序的。因此在这里我们必须定义自己的partition类，保证执行partition过程之后所有Reduce上的数据在整体上是有序的。

具体程序如下所示：

import java.io.IOException;
import org.apache.hadoop.fs.*;
import org.apache.hadoop.conf.Configuration;
import org.apache.hadoop.io.IntWritable;
import org.apache.hadoop.io.Text;
import org.apache.hadoop.mapreduce.lib.input.FileInputFormat;
import org.apache.hadoop.mapreduce.lib.output.FileOutputFormat;
import org.apache.hadoop.mapreduce.Job;
import org.apache.hadoop.mapreduce.Reducer;
import org.apache.hadoop.util.GenericOptionsParser;
import java.net.URI;


import org.apache.hadoop.mapreduce.Mapper;
import org.apache.hadoop.mapreduce.Partitioner;

public class Sort {
	
	public static class Map extends Mapper<Object, Text, IntWritable, IntWritable>{
		private static IntWritable data = new IntWritable();
		public void map(Object key, Text value, Context context) throws IOException, InterruptedException {
			data.set(Integer.parseInt(value.toString()));
			context.write(data, new IntWritable(1));
		}
	}
	
	public static class Reduce extends Reducer<IntWritable, IntWritable, IntWritable, IntWritable> {
		private static IntWritable linenum = new IntWritable(1);
		
		public void reduce(IntWritable key, Iterable<IntWritable> values, Context context) throws IOException, InterruptedException {
			for(IntWritable val: values){
				context.write(linenum, key);
				linenum = new IntWritable(linenum.get()+1);
			}
		}
		
	}
	
	public static class Partition extends Partitioner<IntWritable, IntWritable> {
		@Override
		public int getPartition(IntWritable key, IntWritable value, int partitionNums){
			int MaxNum = 65535;
			int bound = MaxNum/partitionNums+1;
			for(int i=0;i<partitionNums;i++){
				if (key.get()<bound*i && key.get()>bound*(i-1)) {
					return i-1;
				}
			}
			return -1;
		}
		
	}

	public static void main(String[] args) throws Exception{
		Configuration conf = new Configuration();
		String[] remainingArgs = new GenericOptionsParser(conf, args).getRemainingArgs();
		
		if (remainingArgs.length!=2) {
			System.err.println("please input two directories, one is input, the other is output!");
			System.exit(2);
		}
		
		FileSystem fileSystem = FileSystem.get(URI.create(remainingArgs[1]),conf);
		Path path = new Path(remainingArgs[1]);
		if (fileSystem.exists(path)) {
			fileSystem.delete(path, true);
		}
		
		Job job = new Job(conf, "MySort");
		job.setJarByClass(Sort.class);
		job.setMapperClass(Map.class);
		job.setReducerClass(Reduce.class);
		job.setPartitionerClass(Partition.class);
		job.setOutputKeyClass(IntWritable.class);
		job.setOutputValueClass(IntWritable.class);
		FileInputFormat.addInputPath(job, new Path(remainingArgs[0]));
		FileOutputFormat.setOutputPath(job, path);
		System.exit(job.waitForCompletion(true)?0:1);
	}

}

这里的程序为对数字进行的大小排序。